1. Field of the Invention
This invention relates to a multiple step fuel cell seal design which solves the problem of wet seal integrity of commercial size fuel cell stacks, both during the fuel cell conditioning phase and stack operation. The multiple step design reduces or completely eliminates matrix expulsion during fuel cell stack conditioning. The multiple step design of this invention is able to tolerate greater pressure differentials and increase fuel cell gas sealing efficiency during operation. The design is particularly applicable to internal manifolded molten carbonate fuel cell stack systems.
2. Description of Prior Art
Generally, fuel cell electrical output units are comprised of a stacked multiplicity of individual fuel cell units separated by inert or bi-polar electronically conductive separator plates. The separator plates provide the means for directing the reactant gases to the electrodes of the fuel cell units as well as support the cell's internal components. Individual cells are sandwiched together and secured into a single stage unit to achieve the desired fuel cell energy output. Each individual fuel cell unit generally includes an anode and cathode electrode, a common electrolyte, and a fuel and oxidant gas source. Both fuel and oxidant gases are introduced through manifolds to their respective reactant chambers between the separator plate and the electrolyte. The area of contact between the electrolyte and other cell components to maintain separation of the fuel and oxidant gases and prevent and/or minimize gas leakages is known as the wet seal. In addition to supporting the cells components, the wet seal area of the bi-polar separator plates must also provide a seal to prevent reactant gas leakage inside or to the outside of the fuel cell unit or stack. A major factor contributing to premature high temperature fuel cell failure is corrosion and fatigue in the wet seal area. This failure is hastened by corrosive electrolyte contact at high temperatures and high thermal stresses resulting from large temperature variations during thermal cycling of the cell, causing weakening of the structure through intracrystalline and transcrystalline cracking. Such failures permit undesired fuel and/or oxidant gas crossover and overboard gas leakage which interrupts the intended oxidation and reduction reactions of the fuel cell, thereby causing breakdown and eventual stoppage of cell current generation.
Commercially viable fuel cell stacks may contain up to about 600 individual fuel cell units, each having a planer area on the order of 3 to 12 square feet. In stacking such individual cells, separator plates separate the individual cells, with fuel and oxidant being introduced between a set of separator plates, the fuel being introduced between one face of the separator plate and the anode side of an electrolyte and oxidant being introduced between the other face of the separator plate and the cathode side of a second electrolyte. U.S. Pat. No. 5,342,706 to Marianowski et al. teaches a fully internal manifolded fuel cell stack which, upon assembly with electrolyte, has wet seals between the electrolyte and electrodes to provide ease of assembly and long term stability. However, during commercial size fuel cell stack testing of a fuel cell stack of the type disclosed by the '706 patent, we have observed cell matrix material being compressed and extruded outside of the cell areas. During electrolyte melting, under the clamping force applied to the cell stack, the matrix layers thin horizontally in the seal areas by the extrusion of the material laterally. The matrix material, because of the excess of electrolyte caused by the lack of porous electrodes in the wet seal area, becomes very soft and pliable, almost to a semi-plastic phase.
U.S. Pat. No. 3,867,206 to Trocciola et al. teaches a fuel cell having an electrolyte-saturated porous electrode and an electrolyte-saturated matrix between surfaces which are thereby wet by the electrolyte, the electrolyte providing a wet capillary seal against the escape of gas.
A manifold seal structure for fuel cell stacks comprising a mechanical interlock between the adhesive sealing strip and the abutting surface of the gas manifolds is taught by U.S. Pat. No. 4,738,905 to Collins. A phosphoric acid fuel cell having integral edge seals formed by an elastomer permeating an outer peripheral band contiguous with the outer peripheral edges of the cathode and anode assemblies and matrix to form an integral edge seal is taught by U.S. Pat. No. 5,096,786 to Granata, Jr. et al. And densified edge seals for fuel cell components and a method for forming densified edge seals are taught by U.S. Pat. No. 4,365,008 and U.S. Pat. No. 4,269,642, both to DeCasperis et al.
U.S. Pat. No. 4,514,475 to Mientek teaches a separator plate for separating adjacent fuel cells in a stack, which separator plate has two opposite side margins folded back over one side of the plate to form two first seal flanges and the other side margins folded back over the opposite side of the plate to form two second seal flanges. Each of the sealed flanges cooperates with the plate to define a channel in which is disposed a resiliently compressible stack of thin metal sheets.
U.S. Pat. No. 4,609,595 to Nickols teaches a fuel cell separator plate, the center portion of which is a generally rectangular, flat, electrical conductor. Around the periphery of the flat portion of the separator plate, a plurality of elongated resilient flanges are provided which form a gas-tight seal around the edges of the fuel cell. A fuel cell separator having a rectangular flat plate with two unitary upper sealing flanges respectively comprising opposite marginal edges of the plate folded upwardly and back on themselves and two lower sealing flanges respectively comprising the other two marginal edges of the plate folded downwardly and back on themselves is taught by U.S. Pat. No. 4,604,331 to Louis.
U.S. Pat. No. 4,279,970 to Breault et al. teaches a fuel cell comprising an electrolyte matrix layer disposed between and in contact with cooperating anode and cathode electrodes.
A seal for a gas distribution plate of a fuel cell having a groove extending along the edge of the distribution plate in which a resinous material is provided and a paste comprising an immobilized acid is provided surrounding the resinous material, thereby substantially filling the groove, is taught by U.S. Pat. No. 4,450,212 to Feigenbaum et al. The seal, thus formed, is said to be impervious to the gas being distributed throughout the fuel cell. An electrolyte-type seal formed by a bi-polar electrode support structure for a lead-acid battery having a porous matrix of valve metal impregnated with lead or a lead alloy and provided on at least one surface thereof with a rim-portion at which little or no lead is exposed is taught by U.S. Pat. No. 4,178,216 to Nordblom et al.
However, none of the prior art references of which we are aware completely solve the problem of wet seal integrity of commercial size fuel cell stacks both during the fuel cell conditioning phase (that is, start up) and fuel cell stack operation.